Various methods have been tried to date for incorporating lubricating agents into resins with a view to imparting lubricity to such resin. Of the various methods, that which involves the incorporation of a lubricating oil has been particularly effective. However, the conventional techniques invariably involve many drawbacks and none of them have proved to be quite satisfactory from the practical point of view.
For example, Japanese Patent Publication No. 21786/1966 discloses a composition for bearing materials which has a lubricating oil of petroleum origin or a synthetic lubricating oil incorporated in ultra-high molecular weight polyethylene, Japanese Patent Publication No. 5321/1971 discloses a method for adding a lubricating oil to polyacetal or polyamide in the form of powder, and Japanese Patent Publication No. 41092/1972 discloses a composition which has an ester type lubricating oil incorporated in a polycarbonate resin.
In the combination of utra-high molecular weight polyethylene and a petroleum or synthetic lubricating oil, the components have a fairly high affinity for each other, there is a disadvantage that when the lubricating oil is incorporated in a sufficiently large quantity so as to attain ample improvement of the lubricity, the resultant composition becomes conspicuously soft and cannot be used for purposes which call for rigidity. Another disadvantage suffered by this combination is that the lubricating oil exudes and renders the surface of the composition wet and sticky. Since polyacetal, polyamide, or polycarbonate resin has substantially no affinity with lubricating oil, it is difficult to incorporate a sufficiently large quantity of lubricating oil into the resin. Even if a large quantity of lubricating oil is managed to be incorporated by use of a special device or method, the lubricating oil separates from the resin and consequently exudes heavily and renders the surface of the composition wet and sticky. Attempts to mold such compositions by use of an ordinary screw type injection or extrusion molding machine fail because the composition fails to make smooth contact with the interior contour of the molding machine. If there is employed a special device adapted to ensure perfect contact of the composition with the interior of the molding machine, the wet and sticky surface of the composition smears the metal dies while the molding is in process. With these methods, therefore, it has been difficult to obtain molded products of high dimensional accuracy and excellent surface appearance.
With a view to eliminating the drawbacks described above, various methods have been proposed involving the addition of a third substance. None of these studies have so far yield results which are fully satisfactory from the practical point of view. For example, Japanese Patent Publication No. 4634/1953 describes a method in which a lubricating oil solid at normal room temperatures and graphite or a finely divided carbon powder is added to a polyamide type synthetic resin, and Japanese Patent Publication No. 37571/1973 discloses a composition in which a thermoplastic material incorporates a mixture formed by impregnating graphite with a lubricating oil. The solid additives such as graphite, finely divided carbon powder and graphite have only a small capacity for retaining the lubricating oil. When the lubricating oil is added in a large quantity in order to impart sufficient lubricity, the lubricating oil readily separates from the plastic material in the molded article and exudes heavily to render the surface of the product wet and sticky.
Japanese Patent Publication No. 29374/1972 discloses a composition formed by adding to a synthetic resin material a product obtained by mixing ultra-high molecular weight polyethylene with a lubricating oil. The mixture of ultra-high molecular weight polyethylene with the lubricating oil lacks fluidity. When this mixture of insufficient fluidity is added to the synthetic resin material, it is not easily melted and dispersed while the composition is undergoing melt-mixing, extruding and molding processes. Alternatively, the ultra-high molecular weight polyethylene maybe finely divided and intimately mixed with a lubricating agent and thereafter the mixture added to the synthetic resin material, or the ultra-high molecular weight polyethylene maybe impregnated with the lubricating oil, then finely divided and thereafter added to the synthetic resin material. Even with these methods, it is difficult to manufacture a composition in which the ultra-high molecular weight polyethylene containing the lubricating agent is uniformly dispersed in the synthetic resin material, because the particles of the ultra-high molecular weight polyethylene distributed within the synthetic resin material have a particle diameter which is not smaller than that of the particles prior to addition to the resin material. Consequently, it is difficult to obtain from such composition a molded product having high dimensional accuracy and excellent surface appearance. It is similarly difficult to manufacture from such a composition products such as films and fibers.
With a view to improving the abrasion resistance of the composition described in Japanese Patent Publication No. 29374/1972, Japanese Patent Publication No. 37572/1973 discloses a composition which incorporates in addition, as a fourth component, a saturated or unsaturated higher fatty acid, a salt, an ester, an amide or chloride thereof or a metallic soap. Even with the addition of such a fourth component, it is still difficult to overcome the aforementioned drawbacks.
We have proposed a composition formed by combining a thermoplastic resin, a hydrocarbon lubrication oil as the lubricating agent and an ethylene copolymer to absorb and retain the lubricating agent (Japanese Patent Application No. 55677/1971 and Japanese Patent Disclosure No. 22133/1973). This composition has a disadvantage that since the ethylene copolymer has high affinity with the hydrocarbon type lubricating oil and absorbs a large quantity of the lubricating oil at normal room temperatures, the absorbed lubricating oil does not easily separate from the polymer and exude to the surface of the composition.